1. Field of the Invention
The present invention relates to a method and a device for driving a light source, an image reader, and an image forming apparatus.
2. Description of the Related Art
Image readers such as scanners typically employ a light emitting diode (LED) as a light source in recent years. LEDs vary greatly from one LED to another in light quantity emitted therefrom. Accordingly, an image reader generally performs image scanning by emitting light of which quantity is adjusted on an LED-by-LED basis onto a document.
There are two major schemes for adjusting the quantity of light emitted from an LED: an electric-current value control scheme (hereinafter, “current value control scheme”) and a light emission time control scheme (also referred to as pulse width modulation (PWM) scheme). The current value control scheme is disadvantageous in that image quality of an image obtained by scanning is unstable because luminescent chromaticity of an LED greatly depends on an electric current supplied to the LED. Accordingly, image readers generally employ the light emission time control scheme.
Meanwhile, image readers have a function of pre-scanning a document to detect a size in the main-scanning direction of the document. This detection of the size in the main-scanning direction is performed when a user has placed the document on the image reader, such as when a platen for pressing the document is closed. There arises a problem at this time that the user is dazzled by light emitted from an LED onto the document during pre-scanning performed by the image reader.
In this regard, a technique related to an image reader including LEDs as a light source is disclosed in Japanese Patent Application Laid-open No. H8-069215. According to this technique, the quantity of light emitted onto a document during pre-scanning is controlled to be smaller than that during normal document scanning.
However, the conventional technique is disadvantageous in that when the PWM scheme is used to control the light quantity of the LED, a peak value of the light quantity during pre-scanning cannot be reduced, and therefore glare that dazzles the user cannot be reduced. The reason for this is as follows. Although a human eye is dazzled by glare at a peak value of the light quantity of the LED, the user remains to be dazzled by glare of afterimage for a while even after the quantity of light emitted from the LED becomes lower than the peak value.
When the current value control is used to control the quantity of light emitted from the LED, not only image quality of an image obtained by scanning a document is unstable but also luminescent chromaticity of the LED fluctuates. Accordingly, it is substantially impossible to reduce glare that dazzles a user during pre-scanning.
Therefore, there is a need for a device and a method capable of increasing stability of image quality of an image obtained by normal document scanning and also reduce glare that dazzles a user during pre-scanning.